Surface protective films of flexible printed circuits are for example adhesive-bonded polyimide films, called coverlay films, that are punched out with a die conforming to the pattern, and are screen-printed films of UV or heat curable overcoating agents having flexibility, with the latter being more advantageous particularly in workability. Known such curable overcoating resins include resin compositions based on epoxy resins, acrylic resins or mixtures thereof. These compositions are often based on resins modified by introduction of butadiene, siloxane, polycarbonate diol or long chain aliphatic structures, whereby the flexibility is improved and the warpage due to cure shrinkage or heat shrinkage is prevented while minimizing the reduction of heat resistance, chemical resistance and electrical insulating properties inherent to the surface protective films.
With recent weight reduction and miniaturization of electronic equipment, flexible substrates are reduced in weight and thickness and are therefore more significantly susceptible to the flexibility and cure shrinkage of the overcoating resin compositions. Consequently, the curable overcoating resins do not satisfy performance requirements in terms of flexibility and warpage due to cure shrinkage.
JP-A-2004-137370 (Patent Document 1) discloses a polyamideimide resin produced through reaction of a polycarbonate diol from a diol having 6 or less carbon atoms, with a diisocyanate compound to prepare a polyurethane having two isocyanate groups at both ends, and reaction of the diisocyanate-terminated polyurethane with trimellitic acid. Cured products of the resin have unsatisfactory long-term reliability in electrical characteristics.
JP-A-2004-182792 (Patent Document 2) discloses a polyamideimide resin with an organosiloxane structure. Cured products of the resin have bad adhesion to substrates. This prior art uses special solvents such as N-methyl-2-pyrrolidone, which can dissolve emulsifying agents in the screen-printing, often resulting in problems.    Patent Document 1: JP-A-2004-137370    Patent Document 2: JP-A-2004-182792